1. Field of the Invention
The present invention relates to an auto-ionization pumped anti-Stokes Raman laser and, more particularly, to an auto-ionization pumped anti-Stokes Raman laser capable of storing an ionic population inversion in a metastable state, that is, a non-dipole-allowed transition-to-ground state of an ion.
2. Description of the Prior Art
An anti-Stokes Raman laser may be defined as stimulated anti-Stokes Raman emission induced by a pump laser between two levels of the same parity in which a population inversion exists between the upper and lower Raman states. Such laser devices are particularly attractive since they are tunable by tuning the pump laser and, because the upper Raman state is often a metastable level, large inversion densities and high anti-Stokes output energies are possible. Early work in this area is reported in an article entitled "Observation of Stimulated Anti-Stokes Raman Scattering in Inverted Atomic Iodine" by R. L. Carman et al. appearing in Physical Review Letters, Vol. 33, No. 4, July 22, 1974 at pp. 190-193. As described therein, measurable gain in inverted I atoms may be obtained, where the I* (5p.sup.5 2P.sup.0.sub.1/2) state is populated by flash photolysis of trifluoromethyliodide (CF.sub.3 I). The anti-Stokes Raman signal may be observed by pumping this inversion with the fundamental of a Nd:YAC (yttrium aluminum garnet) laser at 1.06 .mu.m and probing with a broadband dye laser. The article goes on to state, however, that superfluorescent emission at the nonresonant anti-Stokes wavelength was not observed during these experiments.
The key to the anti-Stokes Raman laser is the ability to create population inversions between states of the same parity. In many cases, for storage considerations and/or ease of excitation, the creation of a population inversion between states of the same parity is accomplished by creating an inversion in a metastable state with respect to the ground state of the species. Further, to avoid having the stored population drain away, no dipole allowed transitions should exist between the metastable state and the ground state.
For many applications, in particular VUV tunable lasers, auto-ionization pumping to such storage levels in ions is particularly useful. That is, the population may be pumped to a region bove a metastable level, where it will automatically ionize and fall back to the metastable level. Prior work involving auto-ionization techniques was concerned with creating population inversions in dipole-allowed states to ground, as disclosed in the article "Inversion of the Resonance Line of Sr.sup.+ Produced by Optically Pumping Sr Atoms" by W. R. Green et al. appearing in Optics Letters, Vol. 2, No. 5, May 1978 at pp. 115-116. Another article, entitled "Autoionization-Pumped Laser" by J. Bokor et al. appearing in Physical Review Letters, Vol. 48, No. 18, May 3, 1982 at pp. 1242-1245, discusses subsequent work in this area and describes a two-photon pumped inversion into a dipole-allowed state.
The problem with pumping into dipole-allowed states is that the population will naturally drain away into lower states, tending to dissipate the population available for lasing.